Sherman Firefly

Designing the Firefly

In fact, there was just one professional assigned to the project, a Mr W. G. K Kilbourn, who at the time was assigned to the Department of Tank Design at Chertsey, although in normal times, he was a professional engineer employed by Vickers. Not a great deal is known about him, but he must have been a gifted engineer because he tackled and solved the problems that had baffled Brighty; and indeed did it in an ingenious way.

In its regular role as an anti-tank gun, the 17-pounder employed a hydropneumatic recoil system with a travel of 40 inches (101.6cm), which was far too long for the cramped interior of a Sherman turret. Kilbourn therefore decided to replace the existing recoil cylinders with shorter ones. They would be mounted either side of the gun tube, but above and below the centre line of the barrel on opposite sides, all in a special cradle. So far so good, but despite the shorter recoil movement, Kilbourn noticed that the barrel tube was not as well supported as it was on the towed gun because the cradle which he had designed, although totally surrounding the barrel, had inevitably been shortened in order to fit into the turret. The remedy for this was to increase the contact between the barrel and the cradle, and since it was patently impossible to lengthen the cradle, this would mean modifying the barrel tube.

Artillerymen and gun makers have a vocabulary all their own, calling that part of a gun barrel that tapers forwards from the breech the ‘chase’. Kilbourn reckoned that in order to improve contact with the cradle, it was necessary to extend the gun tube at a constant diameter, and in effect, move the chase further forward. This was a major engineering task: a sleeve had to be added to the barrel and then turned on a large lathe to produce the desired result.

If Kilbourn thought that he had solved the problem with these modifications to the gun he was wrong. The ingenuity he had employed to fit the weapon into the tank was commendable, but there were other matters to consider, in particular the crew and the ammunition. The big gun, with a strong recoil guard behind the breech, virtually divided the turret into two halves. This meant that the loader, on the left side of the gun, would find it very difficult to escape in a hurry if the tank were to be hit and blew up. It would mean wriggling under the gun, climbing onto the commander’s seat, and then sliding out through his hatch. The solution was to create a hatch for the loader on the left side, which had another advantage: it made it a lot easier to pass the larger rounds that were required for the big gun into the turret for stowage.

And that was not all. When the US Army adopted the Sherman tank, following the appearance of the prototype T6 in September 1941, they also adopted the British practice of fitting the radio set in the rear of the turret rather than in the hull. The Americans used their SCR 508 set, which was somewhat larger than its British equivalent, although mounting brackets were provided for both. The British No. 19 set, linked with the smaller No. 38 set, was perched at the back of the turret where the loader, who doubled as wireless operator, could reach to manipulate the controls. Whether, as some say, a set in this position would actually be destroyed by the recoil movement of the 17-pounder is open to doubt, but it must have come too close for comfort and certainly too close for the loader to operate it properly.

The solution, which may or may not have been devised by Kilbourn, was to cut a hole in the armour at the back of the turret and then weld on an armoured box, large enough to contain the radios that were dropped in from above, and then covered with a lid. It has to be said that the opening in the armour was hardly a work of art. In the preserved Firefly at the Tank Museum in Bovington, Dorset, it is a relatively small hole, very roughly cut, through which the loader had to reach in order to control the knobs. That box, such an obvious protrusion on the back of the turret, is probably the best way of identifying a Sherman Firefly if you cannot see the gun.